Thrombotic thrombocytopenic purpura (TIP), a disease characterized by von Willebrand factor (VWF)- and platelet-rich thrombi in the arterioles and capillaries, typically presents with thrombocytopenia and microangiopathic hemolysis, accompanied by neurological dysfunction, renal abnormalities and other organ dysfunctions in the advanced cases. The use of plasma exchange has improved the survival rate of the disease from <10% to 80% -90%. However how plasma therapy induces remission was unknown. Studies on the homeostasis of VWF in our laboratory have led to the recognition that VWF is cleaved in the circulation by a plasma metalloprotease in a shear dependent manner and that this process is critical in preventing the accumulation of superactive forms of VWF. Our studies in the last grant cycle discovered that a deficiency of this protease, caused by autoimmune inhibitors, is detected in patients with TTP. Furthermore, investigation of pedigrees with Upshaw-Schulman syndrome reveals that patients with this disorder have hereditary deficiency of the same protease that is transmitted in an autosomally semi-dominant manner. Genomic scans of these pedigrees have mapped the defect to a single locus or chromosome 9q34. Sequence analysis of candidate genes in this region detects mutations in a novel member of the zinc metalloprotease gene family ADAMTS (a disintegrin and metalloprotease with thrombospondin type 1 motif). Expression of the cDNA of this gene, ADAMTS13, shows that it encodes the VWF cleaving metalloprotease. Together these results demonstrate that deficiency of ADAMTS13 causes TTP. This proposal seeks to extend these advances toward improved diagnosis and management of the disease: (1) To test the hypothesis that persistence of inhibitors of ADAMTS13 predisposes the patients to subsequent relapse; (2) To test the hypothesis that an enzyme may decrease the size of VWF to compensate for the lack of ADAMTS13 activity in a knockout mouse model; (3) To identify a protein of the ADAMTS13 sequence that blocks the inhibitors of TTP patients; and (4) To define the genomic regions that regulate the transcription of ADAMTS13. The studies of this proposal will provide data for achieving our goals of preventing the relapse of TTP, facilitating the transition of the ADAMTS13 analysis to clinical practice, exploring the feasibility of developing analogues of ADAMTS13 that may be used to block the inhibitors of ADAMTS13, and devising measures for increasing the expression of ADAMTS13 among patients with hereditary TTP, thereby obviating the need of plasma therapy or factor replacement.